ESTRO 2025 - Abstract Book

S3088

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

2.

GE Healthcare. Milwaukee, WI, USA.

3. A. Cherpak, R. L. MacDonald, and D. Wilke, “HyperSight-ARCHER: Design of a clinical trial to investigate CBCT based adaptive radiotherapy for head and neck cancer.” Seventieth Annual Scientific Meeting of COMP, June 5–8, 2024. Medical Physics. 2024;51(8):5785-5835. 4. Y. Dong et. al, "Metal artifact reduction using virtual monochromatic images for patients with pedicle screws implants on CT.," Eur Spine J., vol. 25, no. 6, pp. 1754-63, 2016. 5. A. T. Davis et. al, "Can CT scan protocols used for radiotherapy treatment planning be adjusted to optimize image quality and patient dose? A systematic review.," Br J Radiol, vol. 90, no. 1076, 2017.

846

Digital Poster Uncertainty in the extent of the internal target volume for lung cancer can be reduced using daily rather than weekly 4D-MRI for adaptive radiotherapy Mark Warren 1 , Lynn Bell 2 , Neeraj Bhalla 2 , David P Cobben 2,3 , Abhishek Mahajan 4,5 , John D Fenwick 6 1 School of Allied Health Professions and Nursing, University of Liverpool, Liverpool, United Kingdom. 2 Clinical Oncology, Clatterbridge Cancer Centre, Liverpool, United Kingdom. 3 Health Data Science, University of Liverpool, Liverpool, United Kingdom. 4 Radiology, Clatterbridge Cancer Centre, Liverpool, United Kingdom. 5 Faculty of Health and Life Sciences, University of Liverpool, Liverpool, United Kingdom. 6 Med Phys & Biomedical Eng, University College London, London, United Kingdom Purpose/Objective: The internal target volume (ITV) is normally determined roughly one week before the first fraction and remains fixed throughout each fraction. However, MR-linac hybrids provide the option to adapt outlines and dose distributions to the daily changes observed in the ITV, using four-dimensional magnetic imaging (4D-MRI) sequences. This adaptive process would consist of a pre-fraction 4D-MRI, followed by a roughly ten-minute adaption period before treatment delivery, where target volumes are redefined, and dosimetric planning is performed. This study investigated if the ITV observed on a daily rather than a weekly 4D-MRI would provide a more accurate representation of the ITV during treatment delivery. Material/Methods: Fourteen patients with stage I-IV lung cancer underwent repeat respiratory-correlated 4D-MRI, using a 2D T2 weighted HASTE sequence. Slices typically covered a region extending from just above the lung apex to just below the diaphragm and were collected in dimensions 1.5 x 1.5 x 5mm 3 , where the 5mm describes the slice thickness. For each patient, repeat image datasets were captured after a ten-minute interval, and at a repeat imaging examination one week later. Repeat image datasets were used to form three types of ITV representing timepoints analogous to daily pre-fraction verification (ITV pre ), treatment delivery ten minutes later (ITV post ), and treatment delivery one week later (ITV week ). To form each ITV, the GTV was first defined on 3-8 phased image stacks. The motion envelope of the GTV on these phased images stacks was then used to outline ITVs on Maximum Intensity Profile images. How well the ITVs correspond with each other was measured using the Hausdorff Distance HD. The maximum distance that either ITV post or ITV week outlines extended outside ITV pre was also calculated. Paired differences in HD were tested for significance using exact permutation testing. Results: Table one shows the mean HD between ITV pre and ITV post is smaller than between ITV pre and ITV week (7.6mm vs 11.1mm, p<0.01). The 90 th percentile of the maximum distances that ITV post extended outside ITV pre was smaller than that for ITV week (9.5mm vs 17.3mm). Figure one shows an example of the differences in ITV extent after a ten-minute interval.